Sensitization

In the context of allergies, sensitization is the process by which a person
becomes, over time, increasingly allergic to a substance (sensitiser) through
repeated exposure to that substance

Source: GreenFacts

More:

The substance may cause a mild response on the first few exposures but, as the
allergy develops, the response becomes worse with subsequent exposures.
Eventually, even short exposures to low concentration can cause very severe
reactions.